(1) Fields of the Invention
The present invention relates to a process for manufacturing seamless metal tubes wherein hollow shells produced by piercing billets are subjected to wall thickness equalization, and subsequently are processed by elongating mill, such as Mannesmann mandrel mill line, Mannesmann plug mill line, Mannesmann multi-stand pipe mill line and Mannesmann assel mill line.
(2) Description of the Prior Art
Seamless metal tubes, and more particularly small-diameter tubes, e.g., tubes 1 to 6 in. in diameter, often involve the problem of wall eccentricity. Concretely, the problem is explained as follows:
The manufacture of small diameter tubes, 1-6 in. in diameter, is conventionally carried out in manner as illustrated in FIG. 6. That is, round bar stock or round billet 10 is heated to 1200.degree.-1250.degree. C. in a rotary hearth furnace 31, pierced by means of a piercing mill 32 (e.g. Mannesmann piercer), and the resulting hollow shell 11 is processed by continuous elongating mill 33 (e.g. a mandrel mill) into a semi-finished pipe 12 having a wall thickness substantially comparable to that of a finished tube. The semi-finished tube 12 is then heated in a reheating furnace (not shown) and sized by means of a stretch reducer 34 to the specified outside diameter. In conjunction with outside diameter sizing, some wall-thickness adjustment is made to obtain the thickness of finished tube. Described above is a typical process for manufacture of small-diameter tubes on a mass-production line known as Mannesmann mandrel mill line. A study of wall eccentricity occurrences under this process revealed that eccectric wall eccentricity in which, as FIG. 7 illustrates, the inside and outside diametral centers do not agree with each other was found with hollow shells 11 from the piercer 32, the wall eccentricity ratio ranging from 5% to 15%. With semi-finished tubes 12 from the elongating mill 33, symmetrical wall eccentricity in which, as FIG. 8 illustrates, the inside and outside diametral centers are identical was found to have occurred in the range of 3 to 5% in terms of wall eccentricity ratio, this eccentricity being added to the above said eccentric wall eccentricity. The term "wall eccentricity ratio" referred to herein is defined as [(Tmax-Tmin)/Tmean].times.100%, wherein Tmax is maximum wall thickness of tube section, Tmin is minimum wall thickness thereof, and Tmean is mean wall thickness thereof.
No appreciable change was caused at the stretch reducer 34. In effect, the wall eccentricity caused at the piercer 32 was introduced into the finished product substantially as it was. It was also discovered that the continuous elongating mill was ineffective for the purpose of wall thickness equalization and that especially where rolling reduction was not uniform in successive passes, some symmetrcal wall eccentricity was added to the initially caused eccentricity.
In such Mannesmann mandrel mill process there is sometimes provided a shell sizer between the piercer 32 and the continuous elongating mill 33. A shell sizer comprises 5 to 7 stands of 2-roll or 3-roll type, each having grooved rolls, arranged in tandem. Each hollow shell 11 is passed through the shell sizer in the axial direction without rotation, so that its outside diameter is reduced to the required outside diameter. It was primarily for the purpose of decreasing the number of sizes of billets to be provided to meet the specifications of various different finished products that the shell sizer was introduced into the Mannesmann mandrel mill process. Granting that only one size of hollow shell is obtainable from one particular size of billet at the piercer 32, the provision of a shell sizer makes it possible to obtain a plurality of shell sizes. It follows that the shell sizer permits simplification of billet sizes and furthers continuous casting of billets. Even if such sizer is applied, however, the wall eccentricity caused in the earlier stage can hardly be corrected: only slight thickness change takes place adjacent the roll flanges, and there is little metal flow in the peripheral direction of the shell.
In the manufacture of medium-diameter tubes, 6 to 16 inches in diameter, a process known as Mannesmann plug mill process is often used which, as FIG. 9 illustrates, comprises a billet 10 heated in a heating furnace 61 being pierced by a piercer 62 into a hollow shell 11, the shell being passed through a rotary elongator 63 for inside diameter expansion or wall thickness reduction, the resulting product being delivered as such to a plug mill 64, then passed through a reeler 65 and a sizer 66 into a finished product. Said rotary elongator 63 is such that a plug is inserted into the hollow shell 11 to perform wall thickness reduction in cooperation with opposed rolls arranged in oblique relation to the shell, so that wall thickness reduction of hollow shell is performed with outer and inner tools under controlled conditions to permit positive metal flow in the peripheral direction for the correction of any wall eccentricity caused at the earlier stage. Insofar as the plug mill process is concerned, there is not much problem of wall eccentricity with medium diameter tubes as is the case with Mannesmann mandrel process in the manufacture of small diameter tubes. Recently, however, there is a growing tendency that a continuous type elongator called "Multi-stand pipe mill," featuring high reduction capacity and high efficiency, is used in the manufacture of medium diameter tubes. Where such mill is combined with above said piercer, the manufacturing line consists of a heating furnace 71, a piercer 72, a multi-stand pipe mill 73 and a sizer 74, as illustrated in FIG. 10. With such simplified arrangement, one similar to that for small-diameter tube manufacturing, there is involved a problem similar to the one noted with small diameter tubes: that any wall eccentricity as caused at the piercer 72 is carried into the finished product. This difficulty may be overcome by providing a rotary elongator between the piercer and the multi-stand pipe mill 73. Indeed, such arrangement is adopted in a known process wherein square bloom is used as stock and wherein a press piercing mill is used in place of an ordinary-type piercer. This arrangement, however, has an economical disadvantage that there are present two different elongators, that is, rotary elongator and multi-stand pipe mill, which fact is unfavorable from the standpoint of equipment investment efficiency.